The present invention relates to a side sill provided on each of a lower right and a lower left portion of the body of an automotive vehicle and, more particularly, to a side sill made of a lightweight material.
An automotive vehicle is necessarily provided with wide openings for accommodating doors at both sides of its body. Since the vehicle body suffers from various kinds of externally derived forces such as vertical bends and longitudinal twists which act around the axis of the vehicle body, a prerequisite is that those portions of the vehicle body which surround the door openings, especially the lower portions, be provided with sufficient rigidity. This prerequisite is generally met by using side sills which are individually located at a lower right and a lower left portions of a vehicle body in such a manner as to extend in the longitudinal direction of the vehicle body. The side sills which serve as reinforcing members for insuring the rigidity of the vehicle body around the door openings as stated above are each provided with a hollow closed cross-section, i.e., a box-like cross-section defined by a top and a bottom wall and a right and a left side wall.
It has been customary to implement a side sill having the above configuration with a sill inner panel and a sill outer panel which are each produced by press-forming a steel sheet having a uniform thickness and provided with flanges at the upper and lower ends thereof. Specifically, the sill inner panel and sill outer panel each having a channel-like configuration are positioned face-to-face and, then, spot-welded together with their upper and lower flanges individually laid one upon the other. To further increase the rigidity as needed, a diagonal reinforcement is put between the upper and lower flanges so as to be spot-welded together with the flanges.
A floor panel interposed between the right and left side sills is usually connected to a side walls of the sill inner panels by spot-welding.
A drawback with the prior art side sill which is made of a steel sheet as stated above is that it has a substantial weight, which obstructs the increasing demand for a lightweight design of a vehicle body that would lead to the cut-down of fuel consumption and others. Another drawback is that the sill inner panel and sill outer panel which are spot-welded together are apt to admit water and others into the side sill, causing the side sill to gather rust. Hence, extra anti-rust measures has to be taken such as providing a seal in each of the joints and/or treating the inner surfaces of the side sill for rust prevention.
In the light of the above, implementing even the side sills which are adapted for the reinforcement of a vehicle body with aluminum, plastics and other lightweight materials has recently come to be discussed. However, in the case that aluminum sheets, for example, are shaped and assembled in the same manner as the traditional steel sheets to produce a side sill, the sill inner panel and sill outer panel cannot be welded together with a sufficient bonding strength due to the property particular to aluminum. Especially, when a diagonal reinforcement is put between the sill inner and sill outer panels which are made of aluminum, it is almost impossible to apply spot-welding to the interconnection of those panels due to the three-layer joint configuration. Adhesive and others which may replace spot-welding stated above would make the sufficient bonding strength even more difficult to achieve.
Further, since aluminum is smaller in the modulus of longitudinal elasticity than steel, an aluminum side sill provided with the same dimensions as the conventional steel side sill would exhibit only insufficient rigidity. Hence, a side sill made of aluminum has to be provided with large cross-sectional dimensions and/or a large wall thickness. However, simply increasing the cross-sectional dimensions of the side sill is undesirable because it reduces the area of the opening available for a door and, thereby, deteriorates the ease of ingress and egress from the passenger compartment. Increasing the thickness of the entire side sill is undesirable also, because such invites an increase in the amount of materail used and, therefore, in the weight of the side sill. Moreover, such a thickness of inner and outer panels further aggravates the difficulty of welding.
In a side sill having a box-like cross-section as described above, stresses are apt to concentrate on the corners defined between the top and bottom walls and the right and left side walls. In addition, since dynamic loads are repeatedly applied to an automotive vehicle while the vehicle is running, substantial stresses are repeately developed in the above-mentioned corners of the side sill. When the side sill is made of aluminum which is relatively susceptible to repetitive loads, cracks are easily produced in the corners of the side sill causing the whole side sill to start breaking at those cracks. This is also true with a side sill which is made of plastics.
For the reasons discussed above, efforts to produce side sills by using lightweight materials have not been made in practice.